In the field of optical lithography for semiconductor industry complex optics are used in order to ensure the required imaging quality. This applies in particular to EUV lithography, i.e. lithography with extreme ultraviolet radiation, in which grazing incidence mirrors and/or multilayer mirrors are arranged in a vacuum chamber between the light source, also referred to as radiation source, and the wafer substrate to be exposed. For developing high power light sources especially for application in EUV lithography the use of metallic radiating material like e.g. Sn and Li is mandatory to reach high efficiencies for the conversion of input power—electrical power for discharge plasma sources or laser power for laser produced plasmas—into EUV radiation.
An implicit problem when using such materials is that these materials will condense on the collector optics which is necessarily placed close to the light source in the same vacuum system to collect the emitted light. The substances or materials released from the light source and moving in the direction of the collector optics are also called debris. Debris layers can deposit on the reflecting surfaces of the collector optics reducing the quality of the optical surfaces. Already very thin deposited layers significantly reduce the reflected intensity of the EUV radiation due to absorption losses on the reflecting surfaces. The amount of material passing from the light source to the reflecting surfaces of the collector optics can be strongly reduced when using debris mitigation devices between the light source and the collector optics, but will never be zero. U.S. 2006/0203965 A1 discloses a device for and method of generating extreme ultraviolet and/or soft X-ray radiation by means of a plasma in which a reduction in the contamination of the optical surfaces is achieved by using an appropriate debris mitigation device.
The deposition of debris will not necessarily be homogeneous over the reflecting surfaces of the optical device, because the emission of theses substances from the radiation source is not necessarily isotropic and also the debris mitigation device has not necessarily the same mitigation efficiencies in all directions. This results in regions on the reflecting surfaces of the collector optics which will degrade faster than others. The operation lifetime of the collector optics will then be shorter compared to a homogeneous deposition because it is defined by the lifetime of the worst region, where the deposition is highest. Already before reaching the end of life criteria for the worst region, the homogeneity of illumination will be affected leading to reduced performance of the illumination system or to reduced throughput if light has to be thrown away to reach homogeneous illumination on a lower level.
Next to deposition of debris material the performance and lifetime of the collector optics is affected by fast particles and ions impinging on the reflecting surfaces of the optical device. This can lead to sputtering of collector material, to roughening or splashing of the coating or to intermixing of adjacent layers if multilayers are used for coating of the collector optics. All these effects lead to reduced reflectivity for the concerned regions of the reflecting surfaces. Also in this case, the exposure of the reflecting surfaces is not homogeneous and some regions might be more affected than others. This leads again to homogeneity problems for the further illumination in the lithographic process and to a reduced throughput and reduced lifetime of the collector optics compared to a homogeneous exposure of the reflecting surfaces.
In order to overcome the above problems the plasma pinch in case of a plasma based light source may be optimized to lead an as much as possible isotropic emission of debris material. Furthermore, the debris mitigation device may be designed to have the same mitigation efficiency for all for the light collection relevant directions or even to have a dedicated design of debris mitigation which corrects for a non isotropic debris emission of the radiation source leading to a homogeneous situation for all the relevant directions after the debris mitigation.